Numerous lean ferritic-austenitic or duplex alloys have been proposed to combat the high costs of raw materials such as nickel and molybdenum with the main goal to accomplish adequate strength and corrosion performance. When referring to the following publications, the element contents are in weight %, if not anything else mentioned.
U.S. Pat. No. 3,736,131 describes an austenitic-ferritic stainless steel with 4-11% Mn, 19-24% Cr, up to 3.0% Ni and 0.12-0.26% N containing 10 to 50% austenite, which is stable and exhibits high toughness. The high toughness is obtained by avoiding austenite transformation to martensite.
U.S. Pat. No. 4,828,630 discloses duplex stainless steels with 17-21.5% Cr, 1 to less than 4% Ni, 4-8% Mn and 0.05-0.15% N that are thermally stable against transformation to martensite. The ferrite content has to be maintained below 60% to achieve good ductility.
EP patent 1327008 describes a lean duplex alloy with high strength, good ductility and high structural stability with 20-23% Cr, 3-8% Mn, 1.1-1.7% Ni and 0.15-0.30% N.
WO patent application 2006/071027 describes a low nickel duplex steel with 19.5-22.5% Cr, 0.5-2.5% Mo, 1.0-3.0% Ni, 1.5-4.5% Mn and 0.15-0.25% N having improved hot ductility compared to similar steels.
EP patent 1352982 disclosed a means of avoiding delayed cracking in austenitic Cr—Mn steels by introducing certain amounts of ferrite phase.
In recent years lean duplex steels have been used to a great extent and steels according to U.S. Pat. No. 4,848,630, EP patent 1327008, EP patent application 1867748 and U.S. Pat. No. 6,623,569 have been used commercially in a large number of applications. Outokumpu LDX 2101® duplex steel according to EP 1327008 has been widely used in storage tanks, transport vehicles, etc. These lean duplex steels have the same problem as other duplex steels, a limited formability which makes them less applicable for use in highly formed parts than austenitic stainless steels. Duplex steels have therefore a limited application in components such as plate heat exchangers. However, lean duplex steels have a unique potential to improved ductility as the austenite phase can be made sufficiently low in the alloy content to be metastable giving increased plasticity by a mechanism as described below.
There are a few references that are utilizing a metastable austenitic phase in duplex steels for improved strength and ductility. U.S. Pat. No. 6,096,441 relates austenitic-ferritic steels with high tensile elongation containing essentially 18-22% Cr, 2-4% Mn, less than 1% Ni and 0.1-0.3% N. A parameter related to the stability in terms of martensite formation shall be within a certain range resulting in improved tensile elongation. US patent application 2007/0163679 describes a very wide range of austenitic-ferritic alloys with high formability mainly by controlling the content of C+N in the austenite phase.
Transformation induced plasticity (TRIP) is a known effect for metastable austenitic steels. For example, local necking in a tensile test sample is hampered by the strain induced transformation of soft austenite to hard martensite conveying the deformation to another location of the sample and resulting in a higher uniform deformation. TRIP can also be used for ferritic-austenitic (duplex) steels if the austenite phase is designed correctly. The classical way to design the austenite phase for a certain TRIP effect is to use established or modified empirical expressions for the austenite stability based on its chemical composition, one of which is the Md30 temperature. The Md30 temperature is defined as the temperature at which 0.3 true strain yields 50% transformation of the austenite to martensite. However, the empirical expressions are established with austenitic steels and there is a risk to apply them on duplex stainless steels.
It is more complex to design the austenite stability of duplex steels since the composition of the austenite phase depends on both the steel chemistry and on the thermal history. Furthermore, the phase morphology and size influence the transformation behaviour. U.S. Pat. No. 6,096,441 has used an expression for the bulk composition and claims a certain range (40-115) which is required to obtain the desired effect. However, this information is only valid for the thermal history used for the steels in this particular investigation, as the austenite composition will vary with the annealing temperature. In US patent application 2007/0163679 the composition of the austenite was measured and a general Md formula for the austenite phase was specified to range from −30 to 90 for steels to show the desired properties.
Empirical formulas for the austenite stability are based on investigations of standard austenitic steels and can have a limited usability for the austenite phase in duplex steel as the conditions for stability are not restricted to the composition only but also to residual stresses and phase or grain parameters. As disclosed in US patent application 2007/0163679, a more direct way is to assess the stability of the martensite by measuring the composition of the austenite phase and then calculate the amount of martensite formation upon cold work. However, this is a very tedious and costly procedure and requires a high class metallurgical laboratory. Another way is to use thermodynamic databases to predict the equilibrium phase balance and compositions of each phase. However, such databases cannot describe the non-equilibrium conditions that prevail after thermo-mechanical treatments in most practical cases. An extensive work with different duplex compositions having a partly metastable austenite phase showed that the annealing temperatures and the cooling rates had a very large influence on the austenite content and the composition making predictions of the martensite formation based on the empirical expressions difficult. To be able to fully control the martensite formation in duplex steels, knowledge of the austenite composition together with micro-structural parameters seemed necessary but not sufficient.